A torque converter having a pump impeller, a turbine wheel, and a stator wheel rotatably disposed between the turbine wheel and the pump impeller is well known in the art. In the toque converter, the stator wheel is coupled to a non-rotary member via a one-way clutch, and the capacity of the torque converter is not variable. Generally, it is desired that the torque converter has a high capacity (capacity coefficient) as its fluid characteristic when it aims at improving the fuel efficiency or reducing fuel consumption. With the above arrangement, however, the capacity of the torque converter is uniquely determined based on the shapes of the pump impeller, turbine wheel and the stator wheel, and the torque, converter exhibits the same fluid characteristics irrespective of the running pattern of the vehicle. Thus, there is a limit to the concurrent improvements of the fuel efficiency and the driving performance.
When the capacity coefficient of the torque converter is high, for example, a difference between the rotational speed of the pump impeller (i.e., the rotational speed of the internal combustion engine) and the rotational speed of the turbine wheel is small. Therefore, if the driver depresses the accelerator pedal while the vehicle is running in a steady state, so as to accelerate the vehicle, the rotational speed of the turbine wheel is not raised unless the transmission is shifted down, and therefore, the driving force cannot be rapidly generated. Thus, where a torque converter having a high capacity is employed, the internal combustion engine is required to be operated in a high-speed low-load region even during steady-state running so that required torque is easily produced when the accelerator pedal is depressed. When the capacity coefficient of the torque converter is low, on the other hand, a difference between the rotational speed of the pump impeller and the rotational speed of the turbine wheel is large; therefore, the response to the depression of the accelerator pedal is improved. In this case, however, a difference between the rotational speed of the pump impeller (pump speed) and the rotational speed of the turbine wheel (turbine speed) is large even during steady-state running of the vehicle, and the internal loss of the torque converter is increased.
In the meantime, a variable capacity type torque converter as described in, for example, Japanese Patent Application Publication No. 01-169170 (JP-A-01-169170) is provided with braking means between the stator wheel and the non-rotary member, and is operable to make its capacity variable by controlling braking torque of the braking means. The control of the braking torque with the braking means makes it possible to change the torque ratio and capacity coefficient of the torque converter continuously (steplessly) or in multiple steps, and the optimum torque ratio and capacity coefficient can be established according to the operating conditions or running conditions, thus assuring improved running performance of the vehicle.
In recent years, the torque converter includes a lock-up clutch engageable to couple the turbine wheel and the pump impeller. By engaging the lock-up clutch when appropriate, the power transmission efficiency of the torque converter is improved. In addition, slip control (flex lock-up control) of the lock-up clutch may be implemented under which a slight slip is caused to occur in the lock-up clutch, so as to enable the lock-up clutch to operate in a wide running or operating region of the vehicle.
If the running region in which the above-mentioned slip control is effected is expanded, the fuel economy or efficiency is generally improved. However, if the slip control is effected in a running region in which the input torque transmitted to the lock-up clutch is large, for example, the amount of heat generated from the lock-up clutch is increased, resulting in a problem of reduction in the durability of the lock-up clutch. In view of this problem, it may be proposed to suppress or prevent the increase of the generated heat amount by reducing a slip amount (a difference in the rotational speed between the turbine wheel and the pump impeller) of the slip control. If the slip amount is reduced, however, a vibration damping effect inherently provided by the torque converter is reduced, which may result in an increase of the booming noise. Thus, the expansion of the slip control region of the lock-up clutch is limited so as to prevent the increase in the generated heat amount of the lock-up clutch and the reduction in the durability.